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symlink.7: Document magic links more completely 

Traditionally, magic links have not been a well-understood topic
in Linux. This helps clarify some of the terminology used in
openat2.2.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Aleksa Sarai 2020-06-10 15:53:19 +10:00 committed by Michael Kerrisk
parent 32b46e5005
commit 41e240557f
1 changed files with 22 additions and 9 deletions
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@ -84,6 +84,21 @@ as they are implemented on Linux and other systems,
are outlined here.
It is important that site-local applications also conform to these rules,
so that the user interface can be as consistent as possible.
.SS Magic-links
There is a special class of symlink-like objects known as "magic-links" which
can be found in certain pseudo-filesystems such as
.BR proc (5)
(examples include
.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .)
Unlike normal symlinks, magic-links are not resolved through
pathname-expansion, but instead act as direct references to the kernel's own
representation of a file handle. As such, these magic-links allow users to
access files which cannot be referenced with normal paths (such as unlinked
files still referenced by a running program.)
.PP
Because they can bypass ordinary
.BR mount_namespaces (7)-based
restrictions, magic-links have been used as attack vectors in various exploits.
.SS Symbolic link ownership, permissions, and timestamps
The owner and group of an existing symbolic link can be changed
using
@ -99,16 +114,14 @@ of a symbolic link can be changed using
or
.BR lutimes (3).
.PP
On Linux, the permissions of a symbolic link are not used
in any operations; the permissions are always
0777 (read, write, and execute for all user categories),
.\" Linux does not currently implement an lchmod(2).
and can't be changed.
(Note that there are some "magic" symbolic links in the
.I /proc
directory tree\(emfor example, the
.IR /proc/[pid]/fd/*
files\(emthat have different permissions.)
On Linux, the permissions of an ordinary symbolic link are not used in any
operations; the permissions are always 0777 (read, write, and execute for all
user categories), and can't be changed.
.PP
However, magic-links do not follow this rule. They can have a non-0777 mode,
though this mode is not currently used in any permission checks.
.\"
.\" The
.\" 4.4BSD